The present invention relates generally to a superconducting generator and, more particularly, to a radiation shield for use with a supercooled rotor.
Superconducting generators typically employ a supercooled rotor which incorporates a superconducting field winding and a rotor structure to support that winding. A fluid refrigerant is used to supercool the field winding to a cryogenic temperature. In typical applications, this refrigerant is helium. The rotor assembly of a superconducting generator also incorporates a pair of torque tubes connected to opposite axial ends of the rotor support structure, a torque tube heat exchanger which is cooperatively associated with each of the torque tubes and a thermal radiation shield which is cylindrically disposed around the field winding portions of the superconducting generator rotor.
When a superconducting generator is in normal operation, a liquid coolant, such as helium, which is contained within a pool near the center of the rotor is converted to a gaseous state through boil-off at a relatively low, but constant, rate. The gaseous coolant is directed toward and along the length of each torque tube by the torque tube's associated heat exchanger. The primary function of the torque tubes is to transmit torque from a prime mover, which is generally at room temperature, to the winding support structure of the superconducting rotor which is at liquid helium temperatures of approximately 4.degree. Kelvin. Also, these torque tubes limit thermal stresses as well as heat leakage which could result due to severe temperature gradients.
The primary function of the radiation shield of a superconducting generator is to intercept heat that is radiated from the rotor's ambient surroundings which are typically at room temperature. The purpose of the radiation shield is to prevent this radiated heat from increasing the temperature of the cryogenic cold zone within the supercooled rotor. In order for the radiation shield to properly inercept the radiated heat from ambient surroundings and remove it from the vicinity of the supercooled field windings, the radiation shield itself must be cooled. However, a properly functioning radiation shield can operate at temperatures which are approximately 100.degree. Kelvin whereas the supercooled field windings themselves must be maintained at a temperature of approximately 4.degree. Kelvin which is below its critical temperature. In order to provide for the supercooling of the radiation shield, a plurality of fluid passages are incorporated within the body of the radiation shield and generally continuous streams of gaseous helium are transported through these fluid passages. U.S. Pat. No. 4,250,418 which issued to Eckels on Feb. 10, 1981 discloses a radiation shield which is used to protect the cold zone of a supercooled rotor from externally radiated heat and is also used to maintain the temperature of the fluid coolant during fault conditions. U.S. Pat. No. 4,319,149 which issued to Eckels on Mar. 9, 1982 discloses a radiation shield which exhibits an improved thermal transient response by including risers disposed within the flow path proximate the discharge end of the fluid passages. The fluid passages of radiation shields are usually provided by a series of grooves in one of two cylinders which are tightly assembled together in coaxial and concentric association.
When a superconducting generator is intended to be used in power plant applications, its design must enable it to survive the most severe operating conditions of such a system which includes a three-phase high voltage transmission line fault. During this type of fault, electromagnetic losses occur in the rotor's field winding, radiation shield and rotor support structure. As a result of these electromagnetic losses, liquid helium within the superconducting rotor is boiled off at a substantially increased rate and the flow rate of helium gas through the torque tube heat exchangers increases substantially and their temperature drops significantly.
During transient faults or other abnormal operating conditions, the flow of helium through the fluid passages of a radiation shield can be sufficiently disturbed so as to cause certain ones of these fluid passages to conduct the gaseous coolant at rates which differ significantly from that of other fluid passages within the radiation shield. Naturally, the portions of the radiation shield which are most proximate these affected fluid passages will be most severely affected by the temperature changes of those passages. In the event that certain passages experience a drastic decrease in coolant flow through them, the radiation shield material surrounding those deprived fluid passages will experience a temperature increase. Conversely, if certain fluid passages within a radiation shield experience an increased flow of coolant through them, the material most proximate those passages will experience a temperature decrease. Under certain contemplated fault conditions and some anticipated steady operating conditions, a combination of both of these deleterious circumstances is possible. Therefore, one portion of the radiation shield can experience an abnormally high temperature while another portion of that radiation shield can simultaneously experience an abnormally low temperature.
As two portions of a radiation shield are experiencing opposing temperature deviations, their respective regions will be thermally affected in such a way so as to cause expansions and contractions of material that can easily cause a significant imbalance in the cylindrical radiation shield. Since typical designs of radiation shields incorporate a plurality of straight and axially extending fluid passageways, a deviation in flow within a specific passageway will affect the material along its path and this affected region will essentially be a strip of material, extending axially from one end of the radiation shield to the other, which is generally straight and parallel to the central axis of the radiation heat shield. This deviation, caused by either an expansion or contraction of material or a combination thereof, will produce an imbalance in a rotating radiation shield and the actual effect of this imbalance will be determined by the distance of this distortion from the center of rotation of the radiation shield. It should be apparent that a combination of an expansion of one portion of a radiation shield along with the contraction of a diametrically opposite portion of the radiation shield could combine to produce a severe distortion and an imbalance of the radiation shield.
The present invention incorporates helical fluid passages extending from one axial end of the radiation shield to the other. These helical fluid passages connect circumferential fluid passages which are located at each axial end of the radiation heat shield. As the helical fluid passages connect and provide fluid communication between the circumferential fluid passages which are at opposite ends of the radiation heat shield, they traverse a helical path which extends around the circumference of the radiation heat shield a preselected number of times. The present invention incorporates a plurality of helical fluid passages which are segregated into subgroups. A radiation shield made in accordance with the present invention typically incorporates four subgroups and each subgroup is associated with an individual circumferential groove at each axial end of the radiation shield. Each subgroup of fluid passages carries the gaseous coolant in an axial direction which is opposite to that of its immediately adjacent subgroups. By alternating the flow direction in this manner, the overall temperature of the radiation shield is held fairly constant along its axial length. By utilizing helical fluid passages, as opposed to straight axial passages, the material of the radiation heat shield which is directly affected by any specific helical passage is distributed in both an axial and circumferential direction. The benefit of the present invention, during fault conditions or other abnormal operations, is that the affected material which is proximate a fluid passage which is operating abnormally is distributed in such a way so as to minimize the net distortion moment about the center of rotation of the radiation shield.
By using helical fluid passages, the present invention avoids serious imbalances which could otherwise be caused by variations in the rate of flow of coolant through different passages. By distributing the potentially affected zones of material around the radiation shield in a helical pattern, the effective net imbalance is reduced significantly and, as the number of helical turns is increased, the imbalance effect of a disturbed coolant flow through any particular fluid passage approaches zero.